8369346: Remove default value of and deprecate the MaxRAM flag

jsikstro · jsikstro · commit 6964cede0269 · 2025-10-29T12:47:18.000Z
Reviewed-by: ayang, lkorinth
diff --git a/src/hotspot/cpu/aarch64/c1_globals_aarch64.hpp b/src/hotspot/cpu/aarch64/c1_globals_aarch64.hpp
@@ -53,7 +53,6 @@ define_pd_global(size_t, CodeCacheExpansionSize,     32*K );
 define_pd_global(size_t, CodeCacheMinBlockLength,    1);
 define_pd_global(size_t, CodeCacheMinimumUseSpace,   400*K);
 define_pd_global(bool, NeverActAsServerClassMachine, true );
-define_pd_global(uint64_t,MaxRAM,                    1ULL*G);
 define_pd_global(bool, CICompileOSR,                 true );
 #endif // !COMPILER2
 define_pd_global(bool, UseTypeProfile,               false);
diff --git a/src/hotspot/cpu/aarch64/c2_globals_aarch64.hpp b/src/hotspot/cpu/aarch64/c2_globals_aarch64.hpp
@@ -55,7 +55,6 @@ define_pd_global(size_t, InitialCodeCacheSize,       2496*K); // Integral multip
 define_pd_global(size_t, CodeCacheExpansionSize,     64*K);
 
 // Ergonomics related flags
-define_pd_global(uint64_t,MaxRAM,                    128ULL*G);
 define_pd_global(intx, RegisterCostAreaRatio,        16000);
 
 // Peephole and CISC spilling both break the graph, and so makes the
diff --git a/src/hotspot/cpu/arm/c1_globals_arm.hpp b/src/hotspot/cpu/arm/c1_globals_arm.hpp
@@ -54,7 +54,6 @@ define_pd_global(size_t, CodeCacheExpansionSize,     32*K );
 define_pd_global(size_t, CodeCacheMinBlockLength,    1);
 define_pd_global(size_t, CodeCacheMinimumUseSpace,   400*K);
 define_pd_global(bool, NeverActAsServerClassMachine, true);
-define_pd_global(uint64_t, MaxRAM,                   1ULL*G);
 define_pd_global(bool, CICompileOSR,                 true );
 #endif // COMPILER2
 define_pd_global(bool, UseTypeProfile,               false);
diff --git a/src/hotspot/cpu/arm/c2_globals_arm.hpp b/src/hotspot/cpu/arm/c2_globals_arm.hpp
@@ -80,9 +80,6 @@ define_pd_global(size_t, NonProfiledCodeHeapSize,    21*M);
 define_pd_global(size_t, ProfiledCodeHeapSize,       22*M);
 define_pd_global(size_t, NonNMethodCodeHeapSize,     5*M );
 define_pd_global(size_t, CodeCacheExpansionSize,     64*K);
-
-// Ergonomics related flags
-define_pd_global(uint64_t, MaxRAM,                   128ULL*G);
 #else
 // InitialCodeCacheSize derived from specjbb2000 run.
 define_pd_global(size_t, InitialCodeCacheSize,       1536*K); // Integral multiple of CodeCacheExpansionSize
@@ -91,8 +88,6 @@ define_pd_global(size_t, NonProfiledCodeHeapSize,    13*M);
 define_pd_global(size_t, ProfiledCodeHeapSize,       14*M);
 define_pd_global(size_t, NonNMethodCodeHeapSize,     5*M );
 define_pd_global(size_t, CodeCacheExpansionSize,     32*K);
-// Ergonomics related flags
-define_pd_global(uint64_t, MaxRAM,                   4ULL*G);
 #endif
 define_pd_global(size_t, CodeCacheMinBlockLength,    6);
 define_pd_global(size_t, CodeCacheMinimumUseSpace,   400*K);
diff --git a/src/hotspot/cpu/ppc/c1_globals_ppc.hpp b/src/hotspot/cpu/ppc/c1_globals_ppc.hpp
@@ -53,7 +53,6 @@ define_pd_global(size_t,   CodeCacheMinBlockLength,      1);
 define_pd_global(size_t,   CodeCacheMinimumUseSpace,     400*K);
 define_pd_global(bool,     NeverActAsServerClassMachine, true);
 define_pd_global(size_t,   NewSizeThreadIncrease,        16*K);
-define_pd_global(uint64_t, MaxRAM,                       1ULL*G);
 define_pd_global(size_t,   InitialCodeCacheSize,         160*K);
 #endif // !COMPILER2
 
diff --git a/src/hotspot/cpu/ppc/c2_globals_ppc.hpp b/src/hotspot/cpu/ppc/c2_globals_ppc.hpp
@@ -86,7 +86,6 @@ define_pd_global(size_t,   NonNMethodCodeHeapSize,       5*M  );
 define_pd_global(size_t,   CodeCacheExpansionSize,       64*K);
 
 // Ergonomics related flags
-define_pd_global(uint64_t, MaxRAM,                       128ULL*G);
 define_pd_global(size_t,   CodeCacheMinBlockLength,      6);
 define_pd_global(size_t,   CodeCacheMinimumUseSpace,     400*K);
 
diff --git a/src/hotspot/cpu/riscv/c1_globals_riscv.hpp b/src/hotspot/cpu/riscv/c1_globals_riscv.hpp
@@ -53,7 +53,6 @@ define_pd_global(size_t, CodeCacheExpansionSize,     32*K );
 define_pd_global(size_t, CodeCacheMinBlockLength,    1);
 define_pd_global(size_t, CodeCacheMinimumUseSpace,   400*K);
 define_pd_global(bool, NeverActAsServerClassMachine, true );
-define_pd_global(uint64_t, MaxRAM,                  1ULL*G);
 define_pd_global(bool, CICompileOSR,                 true );
 #endif // !COMPILER2
 define_pd_global(bool, UseTypeProfile,               false);
diff --git a/src/hotspot/cpu/riscv/c2_globals_riscv.hpp b/src/hotspot/cpu/riscv/c2_globals_riscv.hpp
@@ -55,7 +55,6 @@ define_pd_global(size_t, InitialCodeCacheSize,       2496*K); // Integral multip
 define_pd_global(size_t, CodeCacheExpansionSize,     64*K);
 
 // Ergonomics related flags
-define_pd_global(uint64_t,MaxRAM,                    128ULL*G);
 define_pd_global(intx, RegisterCostAreaRatio,        16000);
 
 // Peephole and CISC spilling both break the graph, and so makes the
diff --git a/src/hotspot/cpu/s390/c1_globals_s390.hpp b/src/hotspot/cpu/s390/c1_globals_s390.hpp
@@ -53,7 +53,6 @@ define_pd_global(size_t,   CodeCacheMinBlockLength,      1);
 define_pd_global(size_t,   CodeCacheMinimumUseSpace,     400*K);
 define_pd_global(bool,     NeverActAsServerClassMachine, true);
 define_pd_global(size_t,   NewSizeThreadIncrease,        16*K);
-define_pd_global(uint64_t, MaxRAM,                       1ULL*G);
 define_pd_global(size_t,   InitialCodeCacheSize,         160*K);
 #endif // !COMPILER2
 
diff --git a/src/hotspot/cpu/s390/c2_globals_s390.hpp b/src/hotspot/cpu/s390/c2_globals_s390.hpp
@@ -74,7 +74,6 @@ define_pd_global(size_t, NonNMethodCodeHeapSize,     5*M);
 define_pd_global(size_t, CodeCacheExpansionSize,     64*K);
 
 // Ergonomics related flags
-define_pd_global(uint64_t, MaxRAM,                   128ULL*G);
 define_pd_global(size_t, CodeCacheMinBlockLength,    4);
 define_pd_global(size_t, CodeCacheMinimumUseSpace,   400*K);
 
diff --git a/src/hotspot/cpu/x86/c1_globals_x86.hpp b/src/hotspot/cpu/x86/c1_globals_x86.hpp
@@ -52,7 +52,6 @@ define_pd_global(size_t, CodeCacheExpansionSize,       32*K );
 define_pd_global(size_t, CodeCacheMinBlockLength,      1    );
 define_pd_global(size_t, CodeCacheMinimumUseSpace,     400*K);
 define_pd_global(bool,   NeverActAsServerClassMachine, true );
-define_pd_global(uint64_t, MaxRAM,                    1ULL*G);
 define_pd_global(bool,   CICompileOSR,                 true );
 #endif // !COMPILER2
 define_pd_global(bool, UseTypeProfile,                 false);
diff --git a/src/hotspot/cpu/x86/c2_globals_x86.hpp b/src/hotspot/cpu/x86/c2_globals_x86.hpp
@@ -52,19 +52,13 @@ define_pd_global(intx,  LoopUnrollLimit,             60);
 // InitialCodeCacheSize derived from specjbb2000 run.
 define_pd_global(size_t, InitialCodeCacheSize,       2496*K); // Integral multiple of CodeCacheExpansionSize
 define_pd_global(size_t, CodeCacheExpansionSize,     64*K);
-
-// Ergonomics related flags
-define_pd_global(uint64_t, MaxRAM,                   128ULL*G);
 #else
 define_pd_global(intx,  InteriorEntryAlignment,      4);
 define_pd_global(size_t, NewSizeThreadIncrease,      4*K);
 define_pd_global(intx,  LoopUnrollLimit,             50);     // Design center runs on 1.3.1
 // InitialCodeCacheSize derived from specjbb2000 run.
 define_pd_global(size_t, InitialCodeCacheSize,       2304*K); // Integral multiple of CodeCacheExpansionSize
 define_pd_global(size_t, CodeCacheExpansionSize,     32*K);
-
-// Ergonomics related flags
-define_pd_global(uint64_t, MaxRAM,                   4ULL*G);
 #endif // AMD64
 define_pd_global(intx, RegisterCostAreaRatio,        16000);
 
diff --git a/src/hotspot/os/windows/os_windows.cpp b/src/hotspot/os/windows/os_windows.cpp
@@ -4160,11 +4160,6 @@ void os::win32::initialize_system_info() {
   }
   _physical_memory = static_cast<physical_memory_size_type>(ms.ullTotalPhys);
 
-  if (FLAG_IS_DEFAULT(MaxRAM)) {
-    // Adjust MaxRAM according to the maximum virtual address space available.
-    FLAG_SET_DEFAULT(MaxRAM, MIN2(MaxRAM, (uint64_t) ms.ullTotalVirtual));
-  }
-
   _is_windows_server = IsWindowsServer();
 
   initialize_performance_counter();
diff --git a/src/hotspot/share/compiler/compiler_globals_pd.hpp b/src/hotspot/share/compiler/compiler_globals_pd.hpp
@@ -72,12 +72,10 @@ define_pd_global(size_t, CodeCacheMinBlockLength,    1);
 define_pd_global(size_t, CodeCacheMinimumUseSpace,   200*K);
 #ifndef ZERO
 define_pd_global(bool, NeverActAsServerClassMachine, true);
-define_pd_global(uint64_t,MaxRAM,                    1ULL*G);
 #else
 // Zero runs without compilers. Do not let this code to force
 // the GC mode and default heap settings.
 define_pd_global(bool, NeverActAsServerClassMachine, false);
-define_pd_global(uint64_t,MaxRAM,                    128ULL*G);
 #endif
 #define CI_COMPILER_COUNT 0
 #else
diff --git a/src/hotspot/share/gc/shared/gc_globals.hpp b/src/hotspot/share/gc/shared/gc_globals.hpp
@@ -268,8 +268,9 @@
   product(bool, AlwaysActAsServerClassMachine, false,                       \
           "Always act like a server-class machine")                         \
                                                                             \
-  product_pd(uint64_t, MaxRAM,                                              \
-          "Real memory size (in bytes) used to set maximum heap size")      \
+  product(uint64_t, MaxRAM, 0,                                              \
+          "(Deprecated) Real memory size (in bytes) used to set maximum "   \
+          "heap size")                                                     \
           range(0, 0XFFFFFFFFFFFFFFFF)                                      \
                                                                             \
   product(bool, AggressiveHeap, false,                                      \
diff --git a/src/hotspot/share/runtime/arguments.cpp b/src/hotspot/share/runtime/arguments.cpp
@@ -536,6 +536,7 @@ static SpecialFlag const special_jvm_flags[] = {
   { "ParallelRefProcEnabled",       JDK_Version::jdk(26),  JDK_Version::jdk(27), JDK_Version::jdk(28) },
   { "ParallelRefProcBalancingEnabled", JDK_Version::jdk(26),  JDK_Version::jdk(27), JDK_Version::jdk(28) },
   { "PSChunkLargeArrays",           JDK_Version::jdk(26),  JDK_Version::jdk(27), JDK_Version::jdk(28) },
+  { "MaxRAM",                       JDK_Version::jdk(26),  JDK_Version::jdk(27), JDK_Version::jdk(28) },
   // --- Deprecated alias flags (see also aliased_jvm_flags) - sorted by obsolete_in then expired_in:
   { "CreateMinidumpOnCrash",        JDK_Version::jdk(9),  JDK_Version::undefined(), JDK_Version::undefined() },
 
@@ -1511,44 +1512,28 @@ static size_t clamp_by_size_t_max(uint64_t value) {
 }
 
 void Arguments::set_heap_size() {
-  uint64_t physical_memory;
-
   // Check if the user has configured any limit on the amount of RAM we may use.
   bool has_ram_limit = !FLAG_IS_DEFAULT(MaxRAMPercentage) ||
                        !FLAG_IS_DEFAULT(MinRAMPercentage) ||
                        !FLAG_IS_DEFAULT(InitialRAMPercentage) ||
                        !FLAG_IS_DEFAULT(MaxRAM);
 
-  if (CompilerConfig::should_set_client_emulation_mode_flags() &&
-      FLAG_IS_DEFAULT(MaxRAM)) {
-    // Reduce the maximum available memory if client emulation mode is enabled.
-    FLAG_SET_DEFAULT(MaxRAM, 1ULL*G);
-  }
-
-  if (has_ram_limit) {
-    if (!FLAG_IS_DEFAULT(MaxRAM)) {
-      // The user has configured MaxRAM, use that instead of physical memory
-      // reported by the OS.
-      physical_memory = MaxRAM;
+  if (FLAG_IS_DEFAULT(MaxRAM)) {
+    if (CompilerConfig::should_set_client_emulation_mode_flags()) {
+      // Limit the available memory if client emulation mode is enabled.
+      FLAG_SET_ERGO(MaxRAM, 1ULL*G);
     } else {
-      // The user has configured a limit, make sure MaxRAM reflects the physical
-      // memory limit that heap sizing takes into account.
-      physical_memory = os::physical_memory();
-      FLAG_SET_ERGO(MaxRAM, physical_memory);
+      // Use the available physical memory on the system.
+      FLAG_SET_ERGO(MaxRAM, os::physical_memory());
     }
-  } else {
-    // If the user did not specify any limit, choose the lowest of the available
-    // physical memory and MaxRAM. MaxRAM is typically set to 128GB on 64-bit
-    // architecture.
-    physical_memory = MIN2(os::physical_memory(), MaxRAM);
   }
 
   // If the maximum heap size has not been set with -Xmx, then set it as
   // fraction of the size of physical memory, respecting the maximum and
   // minimum sizes of the heap.
   if (FLAG_IS_DEFAULT(MaxHeapSize)) {
-    uint64_t min_memory = (uint64_t)(((double)physical_memory * MinRAMPercentage) / 100);
-    uint64_t max_memory = (uint64_t)(((double)physical_memory * MaxRAMPercentage) / 100);
+    uint64_t min_memory = (uint64_t)(((double)MaxRAM * MinRAMPercentage) / 100);
+    uint64_t max_memory = (uint64_t)(((double)MaxRAM * MaxRAMPercentage) / 100);
 
     const size_t reasonable_min = clamp_by_size_t_max(min_memory);
     size_t reasonable_max = clamp_by_size_t_max(max_memory);
@@ -1635,7 +1620,7 @@ void Arguments::set_heap_size() {
     reasonable_minimum = limit_heap_by_allocatable_memory(reasonable_minimum);
 
     if (InitialHeapSize == 0) {
-      uint64_t initial_memory = (uint64_t)(((double)physical_memory * InitialRAMPercentage) / 100);
+      uint64_t initial_memory = (uint64_t)(((double)MaxRAM * InitialRAMPercentage) / 100);
       size_t reasonable_initial = clamp_by_size_t_max(initial_memory);
       reasonable_initial = limit_heap_by_allocatable_memory(reasonable_initial);
 
diff --git a/src/java.base/share/man/java.md b/src/java.base/share/man/java.md
@@ -2566,25 +2566,6 @@ Java HotSpot VM.
 :   Sets the maximum size (in bytes) of the heap for the young generation
     (nursery). The default value is set ergonomically.
 
-`-XX:MaxRAM=`*size*
-:   Sets the maximum amount of memory that the JVM may use for the Java heap
-    before applying ergonomics heuristics. The default value is the maximum
-    amount of available memory to the JVM process or 128 GB, whichever is lower.
-
-    The maximum amount of available memory to the JVM process is the minimum
-    of the machine's physical memory and any constraints set by the environment
-    (e.g. container).
-
-    Specifying this option disables automatic use of compressed oops if
-    the combined result of this and other options influencing the maximum amount
-    of memory is larger than the range of memory addressable by compressed oops.
-    See `-XX:UseCompressedOops` for further information about compressed oops.
-
-    The following example shows how to set the maximum amount of available
-    memory for sizing the Java heap to 2 GB:
-
-    >   `-XX:MaxRAM=2G`
-
 `-XX:MaxRAMPercentage=`*percent*
 :   Sets the maximum amount of memory that the JVM may use for the Java heap
     before applying ergonomics heuristics as a percentage of the maximum amount
@@ -2951,6 +2932,25 @@ they're used.
     (`-XX:+UseParallelGC` or `-XX:+UseG1GC`). Other collectors employing multiple
     threads always perform reference processing in parallel.
 
+`-XX:MaxRAM=`*size*
+:   Sets the maximum amount of memory that the JVM may use for the Java heap
+    before applying ergonomics heuristics. The default value is the amount of
+    available memory to the JVM process.
+
+    The maximum amount of available memory to the JVM process is the minimum
+    of the machine's physical memory and any constraints set by the environment
+    (e.g. container).
+
+    Specifying this option disables automatic use of compressed oops if
+    the combined result of this and other options influencing the maximum amount
+    of memory is larger than the range of memory addressable by compressed oops.
+    See `-XX:UseCompressedOops` for further information about compressed oops.
+
+    The following example shows how to set the maximum amount of available
+    memory for sizing the Java heap to 2 GB:
+
+    >   `-XX:MaxRAM=2G`
+
 ## Obsolete Java Options
 
 These `java` options are still accepted but ignored, and a warning is issued
